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Time to settle up

Unstable ground - A Scandinavian technique for stabilising ground with dry cement injection is being used to renovate flood embankments in east London, reports Adrian Greeman.

Ever since the massed ranks of workers' houses were built in the 1920s for the great Ford motor works at Dagenham, downstream from London, they have been prone to flooding. One of the worst sections is along the valley of the River Beam, a small Thames tributary.

'It is a marsh area and one of the few in Britain with patches below mean sea level, ' explains Mathew Kuhn from consultant Halcrow, which is upgrading flood protection works. 'It means the basin drainage is limited to short periods at very low tide.' To improve things a flood impoundment reservoir was created post-war with two separate 'wing' embankments meeting at the river some way upstream. A sluice gate could be closed to link them and create a reservoir to hold back flood flows until low tide.

But with embankments just 1m high, the Washlands flood storage area was only partially successful. Overtopping was frequent, and local areas, including some 570 houses, regularly flooded. The Ford works were potentially threatened too.

Work to increase the embankments' height to 3m under the Reservoir Act in 1980 improved things but brought new problems.

'The embankments started sinking because of the extra loads when they were built up, ' Kuhn reports. Settlement has generally averaged 10mm-15mm a year, he says, but at times it can speed up suddenly, and 150mm has been recorded within three months in places.

Embankments are now deformed and sunken, and therefore less effective.

A 6m deep layer of peat just below the surface is the problem.

With the additional weight of the new embankments it began to consolidate. There is no prospect of settlement stopping, as peat suffers a long term 'secondary' settlement as pore water is forced out of the ground.

'The organic fibres also rearrange themselves slowly into a more compact structure, ' Kuhn says.

Things came to a head in 2002 when fl oods severely eroded a spillway length of the 600m of embankment. With reservoir fillings becoming more frequent it was clear something had to be done.

Rather than just add to the existing structures, which are composed entirely of clay and therefore prone to cracking when they dry out, new embankments are being built alongside the old ones. Material will be recycled from old to new and mixed with gravel to create what Kuhn calls 'a compactable hogging material'.

The new embankments are being built to the original height, but will be significantly wider at the base. In section they are triangular - 'like Toblerone chocolate bars' - with a narrow 900mm crest.

To prevent the new embankments on 'virgin ground' sinking as the old ones have, it was first decided to overbuild them and allow for forced consolidation over some 18 months.

'But surcharging could have taken up to three years and meant closing the site to the public during that time, ' says Kuhn. Prodded by cost and contract consultant EC Harris, it decided to look at ground improvement instead.

Together with subcontractor Keller Ground Engineering the team explored various 'value engineering' options, primarily forms of ground columns.

Whole-life and risk weighted costings were done to make the comparisons more meaningful.

Stone columns proved too 'wobbly' because of lack of confinement by the soft peat. Vibro-injected concrete columns were better but had problems with various parts of the site where firm underlying gravel gave way to clays and sedimentary material.

The third option looked better.

This was to improve the peat by dry soil mixing, a technique developed in Scandinavia in the last couple of decades but only recently introduced into the UK. Keller saw it being used by Swedish firm LCM and 'liked it so much they bought the company', to steal a famous slogan.

Wet soil mixing has been around for some time but can cause heave problems because of the volume of liquid grout injected; it tends to be used on very robust projects like Boston's Big Dig. But the Swedish method simply mixes in dry powder using a purpose made machine with a mixing head on the end of a mandrel.

About 150kg of cement per cubic metre of fill is injected by a computerised measuring system as the head is forced into the ground; the supply travels in a track-mounted mini-cement silo linked to the main machine by an umbilical cord. 'The driver controls the silo as well as the injector machine, ' says Keller project manager Trevor Costello.

The treatment creates a 'blanket' of firmer ground, which means no load transfer mechanism is needed at ground level.

'This solution firms the peat to a stiff clay-like texture which will see a small amount of settlement in the future, ' says Kuhn. 'We have allowed for about 150mm over 30 years.' During eller's ontract, some 6,000 treated zones, each 800mm in diameter and about 5m-6m long, have been created with two machines. The firmed ground underlies the 12m wide central part of the new embankment, and tapers off on each side to create a total width of 18m.

Term contractor J Breheny prepares the way by forming a temporary bund slightly beyond the existing embankment, using material from the old one. This creates space for the groundwork and then the temporary bund material is used to form layers of clay on the new ground.

Locally won gravel is spread over the clay and the two mixed with a tractor-towed recycling unit usually used for lime stabilisation. The mixture is compacted and built up up to form the new embankment.

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